Steel sheet for hot pressing use, press-formed product, and method for manufacturing press-formed product

ABSTRACT

A steel sheet for hot pressing use according to the present invention has a specified chemical component composition, wherein some of Ti-containing precipitates contained in the steel sheet, each of which having an equivalent circle diameter of 30 nm or less, have an average equivalent circle diameter of 3 nm or more, the precipitated Ti amount and the total Ti amount in the steel fulfill the relationship represented by formula (1) shown below, and the sum total of the fraction of bainite and the fraction of martensite in the metal microstructure is 80 area % or more. 
       Precipitated Ti amount (mass %)−3.4[N]&gt;0.5×[total Ti amount (mass %)−3.4[N]]  (1)
 
     (In the formula (1), [N] represents the content (mass %) of N in the steel.)

TECHNICAL FIELD

The present invention relates to a steel sheet for hot pressing use usedin manufacturing structural components of an automobile and suitable forhot press forming, a press-formed product obtained from such a steelsheet for hot pressing use, and a method for manufacturing thepress-formed product, and relates more specifically to a steel sheet forhot pressing use that is useful in being applied to a hot press formingmethod securing a predetermined strength by being subjected to heattreatment simultaneously with impartation of the shape in forming apre-heated steel sheet (blank) into a predetermined shape, apress-formed product, and a useful method for manufacturing such apress-formed product.

BACKGROUND ART

As one of the fuel economy improvement measures of an automobiletriggered by global environment problems, weight reduction of thevehicle body is advancing, and it is necessary to high-strengthen asteel sheet used for an automobile as much as possible. On the otherhand, when a steel sheet is high-strengthened, shape accuracy in pressforming comes to deteriorate.

On this account, a hot press forming method has been employed formanufacturing components in which a steel sheet is heated to apredetermined temperature (for example, a temperature at which a stateof an austenitic phase is achieved), the strength is lowered, the steelsheet is thereafter formed using a tool of a temperature (roomtemperature for example) lower than the steel sheet, thereby impartationof a shape and rapid heat treatment (quenching) utilizing thetemperature difference of the both are executed simultaneously, and thestrength after forming is secured. Also, such a hot-press forming methodis referred to by various names such as a hot forming method, hotstamping method, hot stamp method, die quench method, and the like inaddition to the hot press method.

FIG. 1 is a schematic explanatory drawing showing a tool configurationfor executing hot press forming described above, 1 in the drawing is apunch, 2 is a die, 3 is a blank holder, 4 is a steel sheet (blank), BHFis a blank holding force, rp is punch shoulder radius, rd is dieshoulder radius, and CL is punch/die clearance respectively. Also, outof these components, in the punch 1 and the die 2, passages 1 a, 2 athrough which a cooling medium (water for example) can pass are formedinside of each, and it is configured that these members are cooled bymaking the cooling medium pass through these passages.

In hot press forming (hot deep drawing for example) using such a tool,forming is started in a state the steel sheet (blank) 4 is heated to atwo-phase zone temperature (between Ac₁ transformation point and Ac₃transformation point) or a single-phase zone temperature of Ac₃transformation point or above and is softened. That is, in a state thesteel sheet 4 in a high temperature state is sandwiched between the die2 and the blank holder 3, the steel sheet 4 is pressed in to the insideof a hole of the die 2 by the punch 1, and is formed into a shapecorresponding to the shape of the outer shape of the punch 1 whilereducing the outside diameter of the steel sheet 4. Also, by cooling thepunch 1 and the die 2 in parallel with forming, heat removal from thesteel sheet 4 to the tools (the punch 1 and the die 2) is executed,holding and cooling are further executed at a forming bottom dead point(the temporal point the tip of the punch is positioned at the deepestpoint: the state shown in FIG. 1), and thereby quenching of the rawmaterial is executed. By executing such a forming method, a formedproduct of 1,500 MPa class with excellent dimensional accuracy can beobtained, the forming load can be reduced compared with a case acomponent of a same strength class is cold-formed, and therefore lesscapacity of the press machine is needed.

As a steel sheet for hot pressing use widely used at present, one using22Mn—B5 steel as a raw material is known. The steel sheet has thetensile strength of approximately 1,500 MPa and the elongation ofapproximately 6-8%, and is applied to a shock resistant member (a membernot causing deformation as much as possible and not causing breakage incollision). However, application to a component requiring deformationsuch as an energy absorption member is difficult because elongation(ductility) is low.

As a steel sheet for hot pressing use exerting excellent elongation,technologies such as the patent literatures 1-4 for example have alsobeen proposed. According to these technologies, the basic strength classof each steel sheet is adjusted by setting the carbon content in thesteel sheet to various ranges, and elongation is improved by introducingferrite with high deformability and reducing the average grain size offerrite and martensite. Although these technologies are effective inimproving elongation, they are still insufficient from the viewpoint ofimproving elongation matching the strength of the steel sheet. Forexample, those having 1,470 MPa or more of the tensile strength TS havethe elongation EL of approximately 10.2% at the maximum, and furtherimprovement is required.

On the other hand, even the formed product having a lower strength classcompared to the hot stamp formed product, 980 MPa class or 1,180 MPaclass of the tensile strength TS for example, having been studied untilnow has a problem in forming accuracy of cold press forming, and thereare needs for low-strength hot press forming as an improvement measuretherefor. At that time, it is necessary to largely improve energyabsorption properties in the formed product.

Particularly, in recent years, development of the technology fordifferentiating the strength within a single component is proceeding. Assuch a technology, a technology has been proposed in which the portionthat must be prevented from deforming has high strength (high strengthside: shock resistant portion side), and the portion that needs energyabsorption has low strength and high ductility (low strength side:energy absorption portion side). For example, in a passenger car of themiddle class or above, there is a case that portions having bothfunctions of shock resistant property and energy absorption property areprovided within a component of a B-pillar and rear side memberconsidering compatibility in a side collision and a rear collision (afunction for protecting the counterpart side also when a small-sized carcollides with). For the purpose of manufacturing such members, (a) amethod of joining a steel sheet becoming of low strength even in beingheated to a same temperature and tool-quenched to a normal steel sheetfor hot pressing use (tailored weld blank: TWB), (b) a method fordifferentiating the strength for each region of a steel sheet bydifferentiating the cooling rate in the tool, (c) a method fordifferentiating the strength by differentiating the heating temperaturefor each region of a steel sheet, and the like have been proposed.

Although the tensile strength: 1,500 MPa class is achieved on the highstrength side (shock resistant portion side) according to thesetechnologies, the maximum tensile strength is 700 MPa and the elongationEL is approximately 17% on the low strength side (energy absorptionportion side), and achievement of higher strength and higher ductilityare required in order to further improve the energy absorptionproperties.

CITATION LIST Patent Literature

-   [Patent Literature 1] JP-A 2010-065292-   [Patent Literature 2] JP-A 2010-065293-   [Patent Literature 3] JP-A 2010-065294-   [Patent Literature 4] JP-A 2010-065295

SUMMARY OF INVENTION Technical Problems

The present invention has been developed in view of such circumstancesas described above, and its object is to provide a steel sheet for hotpressing use capable of obtaining a hot press-formed product that canachieve the balance of high strength and elongation with a high levelwhen uniform property is required within a formed product and is usefulin obtaining a press-formed product that can achieve the balance of highstrength and elongation with a high level according to each region whenregions corresponding to a shock resistant portion and an energyabsorption portion are required within a single formed product, apress-formed product exerting the properties described above, and auseful method for manufacturing such a hot press-formed product.

Solution to Problems

The steel sheet for hot pressing use of the present invention whichcould achieve the object described above contains:

C: 0.15-0.5% (means mass %, hereinafter the same with respect to thechemical component composition);

Si: 0.2-3%;

Mn: 0.5-3%;

P: 0.05% or less (exclusive of 0%);

S: 0.05% or less (exclusive of 0%);

Al: 0.01-1%;

B: 0.0002-0.01%;

Ti: 3.4[N]+0.002% or more and 3.4[N]+0.1% or less ([N] expresses Ncontent (mass %)), and

N: 0.001-0.01% respectively, with the remainder consisting of iron andinevitable impurities, in which

some of Ti-containing precipitates contained in the steel sheet, each ofwhich having an equivalent circle diameter of 30 nm or less, have anaverage equivalent circle diameter of 3 nm or less, the precipitated Tiamount and the total Ti amount in the steel fulfill the relationshiprepresented by formula (1) shown below, and the sum total of thefraction of bainite and the fraction of martensite in the metalmicrostructure is 80 area % or more. Also, “equivalent circle diameter”is the diameter of an imaginary circle having an area same to the size(area) of Ti containing precipitates (TiC for example) (“the averageequivalent circle diameter” is the average value thereof).

Precipitated Ti amount (mass %)−3.4[N]>0.5×[(total Ti amount (mass%))−3.4[N]]  (1)

(In the formula (1), [N] represents the content (mass %) of N in thesteel.)

In the steel sheet for hot pressing use of the present invention,according to the necessity, it is also useful to contain, as otherelements, (a) at least one element selected from the group consisting ofV, Nb and Zr by 0.1% or less (exclusive of 0%) in total, (b) at leastone element selected from the group consisting of Cu, Ni, Cr and Mo by1% or less (exclusive of 0%) in total, (c) at least one element selectedfrom the group consisting of Mg, Ca and REM by 0.01% or less (exclusiveof 0%) in total, and the like, and the properties of the press-formedproduct is improved further according to the kind of the elementscontained.

The method for manufacturing a press-formed product of the presentinvention which could achieve the object described above includes thesteps of using such a steel sheet for hot pressing use of the presentinvention as described above, heating the steel sheet to a temperatureof Ac₁ transformation point+20° C. or above and Ac₃ transformationpoint−20° C. or below, thereafter starting press forming, and executingcooling to a temperature or below, the temperature being lower than thebainite transformation starting temperature Bs by 100° C., whilesecuring the average cooling rate of 20° C./s or more within a toolduring forming and after completion of forming.

In the press-formed product obtained by the method for manufacturing,the metal microstructure includes retained austenite: 3-20 area %,annealed martensite and/or annealed bainite: 30-87 area %, andmartensite as quenched: 10-67 area %, the amount of carbon in theretained austenite is 0.60% or more, and the balance of high strengthand elongation can be achieved with a high level and as a uniformproperty within the formed product. Also, the area ratio of annealedmartensite and/or annealed bainite means the total area ratio of bothmicrostructures when both microstructures are included, and means, wheneither one microstructure is included, the area ratio of themicrostructure.

Also, another method for manufacturing a press-formed product of thepresent invention which could achieve the object described aboveincludes the steps of using such a steel sheet for hot pressing use ofthe present invention as described above, dividing a heating region ofthe steel sheet into two regions, heating one region thereof to atemperature of Ac₃ transformation point or above and 950° C. or below,heating the other region to a temperature of Ac₁ transformationpoint+20° C. or above and Ac₃ transformation point−20° C. or below,thereafter starting press forming, and executing cooling to atemperature of martensite transformation starting temperature Ms orbelow while securing the average cooling rate of 20° C./s or more withina tool during forming and after completion of forming.

In the press-formed product obtained by the method for manufacturing, afirst region whose metal microstructure includes retained austenite:3-20 area % and martensite: 80 area % or more and a second region whosemetal microstructure includes retained austenite: 3-20 area %, annealedmartensite and/or annealed bainite: 30-87 area %, and martensite asquenched: 10-67 area % with the amount of carbon in the retainedaustenite being 0.60% or more are included, the balance of high strengthand elongation can be achieved with a high level according to eachregion, and regions corresponding to a shock resistant portion and anenergy absorption portion are present within a single formed product.

Advantageous Effects of Invention

According to the present invention, because a steel sheet is used inwhich the chemical component composition is strictly stipulated, thesize of Ti-containing precipitates is controlled, the precipitation rateis controlled for Ti that does not form TiN, and the ratio of temperedhard phase (martensitic phase, bainitic phase and the like), hard phase(as-quenched martensite phase) and retained austenite phase is adjustedwith respect to the metal microstructure, by hot-pressing the steelsheet under a predetermined condition, high strength-elongation balanceof the press-formed product can be made a high level. Also, whenhot-pressing is executed under different conditions in plural regions,the shock resistant portion and the energy absorption portion can beformed within a single formed product, the balance of high strength andelongation can be achieved with a high level for each portion.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic explanatory drawing showing a tool configurationfor executing hot press forming.

DESCRIPTION OF EMBODIMENTS

The present inventors carried out studies from various aspects in orderto achieve such a steel sheet for hot pressing use that can obtain apress-formed product exhibiting excellent ductility (elongation) alsowhile securing high strength after press-forming in manufacturing thepress-formed product by heating a steel sheet to a predeterminedtemperature and thereafter executing hot press forming.

As a result of the studies, it was found out that, when the chemicalcomponent composition of the steel sheet for hot pressing use wasstrictly stipulated, the size of Ti-containing precipitates andprecipitated Ti amount were controlled and the metal microstructure wasmade an appropriate one, by hot press forming of the steel sheet under apredetermined condition, a press-formed product in which retainedaustenite of a predetermined amount was secured after press forming andintrinsic ductility (residual ductility) was enhanced could be obtained,and the present invention was completed.

In the steel sheet for hot pressing use of the present invention, it isnecessary to strictly stipulate the chemical component composition, andthe reasons for limiting the range of each chemical component are asfollows.

[C: 0.15-0.5%]

C is an important element in achieving the balance of high strength andelongation of a case uniform properties are required within a formedproduct with a high level or in securing retained austenite particularlyin the low strength/high ductility portion of a case the regionscorresponding to a shock resistant portion and an energy absorptionportion are required within a single formed product. Also, byconcentration of C to austenite in heating of hot press forming,retained austenite can be formed after quenching. Also, C contributes toincrease of the amount of martensite, and increases the strength. Inorder to exert such effects, C content should be 0.15% or more.

However, when C content becomes excessive and exceeds 0.5%, two phasezone heating range becomes narrow, and the balance of high strength andelongation of a case uniform properties are required within a formedproduct is not achieved with a high level, or it becomes hard to adjustthe metal microstructure to that targeted particularly in the lowstrength/high ductility portion (a microstructure in which apredetermined amount of annealed martensite and/or annealed bainite issecured) of a case the regions corresponding to a shock resistantportion and an energy absorption portion are required within a singleformed product. Preferable lower limit of C content is 0.17% or more(more preferably 0.20% or more), and more preferable upper limit is0.45% or less (further more preferably 0.40% or less).

[Si: 0.2-3%]

Si exerts an effect of forming retained austenite by suppressing thatmartensite is tempered during cooling of tool-quenching and cementite isformed, or that untransformed austenite is disintegrated. In order toexert such an effect, Si content should be 0.2% or more. Also, when Sicontent becomes excessive and exceeds 3%, ferrite is liable to beformed, formation of single-phase microstructure becomes hard inheating, and required fractions of bainite and martensite cannot besecured in a steel sheet for hot pressing use. Preferable lower limit ofSi content is 0.5% or more (more preferably 1.0% or more), andpreferable upper limit is 2.5% or less (more preferably 2.0% or less).

[Mn: 0.5-3%]

Mn is an element effective in enhancing quenchability and suppressingformation of a microstructure (ferrite, pearlite, bainite and the like)other than martensite and retained austenite during cooling oftool-quenching. Also, Mn is an element stabilizing austenite, and is anelement contributing to increase of retained austenite amount. In orderto exert such effects, Mn should be contained by 0.5% or more. AlthoughMn content is preferable to be as much as possible when only propertiesare considered, because the cost of adding alloy increases, Mn contentis made 3% or less. Preferable lower limit of Mn content is 0.7% or more(more preferably 1.0% or more), and preferable upper limit is 2.5% orless (more preferably 2.0% or less).

[P: 0.05% or Less (Exclusive of 0%)]

Although P is an element inevitably included in steel, because Pdeteriorates ductility, P is preferable to be reduced as much aspossible. However, because extreme reduction causes increase of thesteel making cost and to make it 0% is difficult in manufacturing, Pcontent is made 0.05% or less (exclusive of 0%). Preferable upper limitof P content is 0.045% or less (more preferably 0.040% or less).

[S: 0.05% or Less (Exclusive of 0%)]

Similar to P, S is also an element inevitably included in steel, Sdeteriorates ductility, and therefore S is preferable to be reduced asmuch as possible. However, because extreme reduction causes increase ofthe steel making cost and to make it 0% is difficult in manufacturing, Scontent is made 0.05% or less (exclusive of 0%). Preferable upper limitof S content is 0.045% or less (more preferably 0.040% or less).

[Al: 0.01-1%]

Al is useful as a deoxidizing element, fixes solid-solution N present insteel as AlN, and is useful in improving ductility. In order toeffectively exert such an effect, Al content should be 0.01% or more.However, when Al content becomes excessive and exceeds 1%, Al₂O₃ isformed excessively, and ductility is deteriorated. Also, preferablelower limit of Al content is 0.02% or more (more preferably 0.03% ormore), and preferable upper limit is 0.8% or less (more preferably 0.6%or less).

[B: 0.0002-0.01%]

B is an element contributing to prevention of formation of ferrite,pearlite and bainite during cooling after heating to a two-phase zonetemperature of (Ac₁ transformation point-Ac₃ transformation point)because B has an action of suppressing ferrite transformation, pearlitetransformation and bainite transformation on the high strength portionside, and to secure retained austenite. In order to exert such effects,B should be contained by 0.0002% or more, however, even when B iscontained excessively exceeding 0.01%, the effects saturate. Preferablelower limit of B content is 0.0003% or more (more preferably 0.0005% ormore), and preferable upper limit is 0.008% or less (more preferably0.005% or less).

[Ti: 3.4[N]+0.01% or More and 3.4[N]+0.1% or Less: [N] Expresses NContent (Mass %)]

Ti develops improvement effect of quenchability by fixing N and holdingB in a solid solution state. In order to exert such an effect, it isimportant to contain Ti more than the stoichiometric ratio of Ti and N(3.4 times of N content) by 0.01% or more. However, when Ti contentbecomes excessive to be more than 3.4[N]+0.1%, Ti-containingprecipitates formed are finely dispersed and impede the growth ofmartensite during cooling after heating to the two phase zonetemperature, a lath (lath-like martensite) with a small aspect ratio isformed, discharging of carbon (C) to retained austenite between thelaths becomes slow, and the carbon amount in the retained austenitereduces. Preferable lower limit of Ti content is 3.4[N]+0.02% or more(more preferably 3.4[N]+0.05% or more), and preferable upper limit is3.4[N]+0.09% or less (more preferably 3.4[N]+0.08% or less).

[N: 0.001-0.01%]

N is an element inevitably mixed in and is preferable to be reduced,however, because there is a limit in reducing N in an actual process,0.001% is made the lower limit. Also, when N content becomes excessive,the ductility deteriorates because of time aging, N precipitates as BN,the quenchability improvement effect by solid-dissolved B isdeteriorated, and therefore the upper limit is made 0.01%. Preferableupper limit of N content is 0.008% or less (more preferably 0.006% orless).

The basic chemical composition in the steel sheet for hot pressing useof the present invention is as described above, and the remainder isiron and inevitable impurities other than P, S (0, H and the like forexample). Further, in the steel sheet for hot pressing use of thepresent invention, according to the necessity, it is also useful tofurther contain (a) at least one element selected from the groupconsisting of V, Nb and Zr by 0.1% or less (exclusive of 0%) in total,(b) at least one element selected from the group consisting of Cu, Ni,Cr and Mo by 1% or less (exclusive of 0%) in total, (c) at least oneelement selected from the group consisting of Mg, Ca and REM (rare earthelements) by 0.01% or less (exclusive of 0%) in total, and the like, andthe properties of the steel sheet for hot pressing use are improvedfurther according to the kind of the element contained. Preferable rangewhen these elements are contained and reasons for limiting the range areas follows.

[At Least One Element Selected from the Group Consisting of V, Nb and Zrby 0.1% or Less (Exclusive of 0%) in Total]

V, Nb and Zr have effects of forming fine carbide and miniaturizing themicrostructure by a pinning effect. In order to exert such effects, itis preferable to contain them by 0.001% or more in total. However, whenthe content of these elements becomes excessive, coarse carbide isformed and becomes a start point of breakage, and ductility isdeteriorated adversely. Therefore, it is preferable to contain theseelements by 0.1% or less in total. More preferable lower limit of thecontent of these elements in total is 0.005% or more (further morepreferably 0.008% or more), and more preferable upper limit in total is0.08% or less (further more preferably 0.06% or less).

[At Least One Element Selected from the Group Consisting of Cu, Ni, Crand Mo: 1% or Less (Exclusive of 0%) in Total]

Cu, Ni, Cr and Mo suppress ferrite transformation, pearlitetransformation and bainite transformation, therefore prevent formationof ferrite, pearlite and bainite during cooling after heating, and acteffectively in securing retained austenite. In order to exert sucheffects, it is preferable to contain them by 0.01% or more in total.Although the content is preferable to be as much as possible when onlythe properties are considered, because the cost for adding alloysincreases, 1% or less in total is preferable. Also, because there is anaction of largely increasing the strength of austenite, the load of hotrolling increases, manufacturing of the steel sheet becomes difficult,and therefore 1% or less is also preferable from the viewpoint ofmanufacturability. More preferable lower limit of these elements intotal is 0.05% or more (further more preferably 0.06% or more), and morepreferable upper limit in total is 0.5% or less (further more preferably0.3% or less).

[At Least One Element Selected from the Group Consisting of Mg, Ca andREM by 0.01% or Less (Exclusive of 0%) in Total]

Because these elements miniaturize inclusions, they act effectively inimproving ductility. In order to exert such effects, it is preferable tocontain them by 0.0001% or more in total. Although the content ispreferable to be as much as possible when only the properties areconsidered, because the effects saturate, 0.01% or less in total ispreferable. More preferable lower limit of these elements in total is0.0002% or more (further more preferably 0.0005% or more), and morepreferable upper limit in total is 0.005% or less (further morepreferably 0.003% or less).

In the steel sheet for hot pressing use of the present invention, (A)some of Ti-containing precipitates contained in the steel sheet, each ofwhich having an equivalent circle diameter of 30 nm or less, have anaverage equivalent circle diameter of 3 nm or less, (B) relationship ofprecipitated Ti amount (mass %)−3.4[N]>0.5×[(total Ti amount (mass%))−3.4[N]] (the relationship of the formula (1) described above) isfulfilled, and (C) the metal microstructure contains at least either oneof bainite and martensite, and the sum total of the fraction of bainiteand the fraction of martensite is 80 area % or more, are also importantrequirements.

When Ti that is excessive with respect to N is dispersed finely ormajority thereof is present in a solid solution state in the steel sheetbefore hot press forming, much amount of Ti comes to be present while itis fine in heating of hot press forming. Thus, in martensitetransformation that occurs during rapid cooling within the tool afterheating, the growth of martensite lath in the longitudinal direction isimpeded, the growth in the width direction is promoted, and the aspectratio reduces. As a result, discharge of carbon from the martensite lathto surrounding retained austenite delays, the carbon amount in retainedaustenite reduces, the stability of retained austenite deteriorates, andtherefore the improvement effect of the elongation cannot be obtainedsufficiently.

From such a viewpoint, Ti-containing precipitates should be dispersedcoarsely, and, for that purpose, it is necessary that some ofTi-containing precipitates contained in the steel sheet, each of whichhaving an equivalent circle diameter of 30 nm or less, have an averageequivalent circle diameter of 3 nm or more (the requirement of (A)described above). Also, the reason the equivalent circle diameter of theTi-containing precipitates of the object is stipulated to be 30 nm orless is that it is necessary to control the Ti-containing precipitatesand excluding TiN formed coarsely in the melting stage that does notaffect microstructure change and properties thereafter. The size of theTi-containing precipitates (the average equivalent circle diameter ofthe Ti-containing precipitates whose equivalent circle diameter is 30 nmor less) is preferably 5 nm or more, more preferably 10 nm or more.Further, the Ti-containing precipitates of the object of the presentinvention also include precipitates containing Ti such as TiVC, TiNbC,TiVCN, TiNbCN and the like in addition to TiC and TiN.

Also, in the steel sheet for hot pressing use, it is necessary that, outof Ti, majority of Ti other than that used for precipitating and fixingN is present in the precipitated state. For that purpose, it isnecessary that the Ti amount present as the precipitates other than TiN(that is, precipitated Ti amount (mass %)−3.4[N]) is more than 0.5 timesof the balance obtained by deducting Ti that forms TiN from total Ti(that is, more than 0.5×[total Ti amount (mass %)−3.4[N]]) (therequirement of (B) described above). Precipitated Ti amount (mass%)−3.4[N] is preferably 0.6×[total Ti amount (mass %)−3.4[N]] or more,more preferably 0.7×[total Ti amount (mass %)−3.4[N]] or more.

Although control of the metal microstructure is intrinsically necessaryfor achieving desired strength-elongation balance in the formed product,the metal microstructure cannot be controlled only by the hot pressingcondition, and it is necessary to control the microstructure of the rawmaterial steel thereof (the steel sheet for hot pressing use)beforehand. In order to secure the proper amount of annealed martensiteand annealed bainite which are fine and largely contributing toductility in the press forming steel sheet, it is necessary to make thesum total of the fraction of bainite and the fraction of martensite inthe steel sheet 80 area % or more. When the sum total of the fraction ofbainite and the fraction of martensite is less than 80 area %, thefraction of annealed martensite and/or annealed bainite targeted ishardly secured, and the amount of other microstructure (ferrite forexample) increases to deteriorate the strength-elongation balance. Thesum total of the fraction of bainite and the fraction of martensite ispreferably 90 area % or more, more preferably 95 area % or more.

Further, in the steel sheet for hot pressing use of the presentinvention, although the remainder of the metal microstructure is notparticularly limited, at least any of ferrite, pearlite or retainedaustenite can be cited for example.

The steel sheet (the steel sheet for hot pressing use) of the presentinvention as described above can be manufactured by that a billetobtained by melting steel having the chemical component composition asdescribed above is subjected to hot rolling with the heatingtemperature: 1,100° C. or above (preferably 1,150° C. or above) and1,300° C. or below (preferably 1,250° C. or below) and the finishrolling temperature of 750° C. or above (preferably 780° C. or above)and 850° C. or below (preferably 830° C. or below), cooling thereafter(slow cooling: intermediate cooling) so as to stay for 10 s or more(preferably 50 s or more) between 700-750° C. (preferably 720-740° C.),cooling (rapid cooling) thereafter to 450° C. or below (preferably 350°C. or below) at 20° C./s or more (preferably 30° C./s or more), andwinding at 100° C. or above (preferably 150° C. or above) and 450° C. orbelow (preferably 400° C. or below).

The method described above is for executing control so that (1) rollingis finished at a temperature range where dislocation introduced by hotrolling remains within austenite, (2) Ti-containing precipitates such asTiC and the like are formed finely on the dislocation by rapid coolingimmediately thereafter, and (3) bainite transformation or martensitetransformation is caused by rapid cooling and winding thereafter.

The steel sheet for hot pressing use having the chemical componentcomposition, metal microstructure and Ti-precipitation state asdescribed above may be used as it is for manufacturing by hot pressforming, and may be subjected to cold rolling with the draft: 10-80%(preferably 20-70%) after pickling. Further, the steel sheet for hotpressing use or the material obtained by cold rolling thereof may besubjected to such heat treatment of heating to such a temperature rangewhere TiC is not dissolved by 100% (1,000° C. or below: for example870-900° C.), rapidly cooling thereafter to 450° C. or below (preferably400° C. or below) at a cooling rate of 20° C./s or more (preferably 30°C./s or more), and holding thereafter at 450° C. or below for 10 s ormore and 1,000 s or less or tempering at a temperature of 450° C. orbelow. Also, the steel sheet for hot pressing use of the presentinvention may be subjected to plating containing at least one elementout of Al, Zn, Mg and Si on the surface thereof (the surface of the basesteel sheet).

By using the steel sheet for hot pressing use as described above,executing heating to a temperature of Ac₁ transformation point+20° C. orabove and Ac₃ transformation point−20° C. or below, thereafter startingpress-forming, and executing cooling to a temperature or below, thetemperature being lower than the bainite transformation startingtemperature Bs by 100° C., while securing the average cooling rate of20° C./s or more within the tool during forming and after completion offorming, the press formed product having a single property (may behereinafter referred to as “single region formed product”) can have anoptimum microstructure of low strength and high ductility. The reasonsfor stipulating each requirement in this forming method are as describedbelow.

In order to form austenite between laths of martensite and bainitewithin the steel sheet and to form annealed martensite and annealedbainite excellent in ductility by annealing martensite and bainite, theheating temperature should be controlled to a predetermined range. Whenthe heating temperature of the steel sheet is below Ac₁ transformationpoint+20° C., sufficient amount of austenite cannot be secured inheating, and a predetermined amount of retained austenite cannot besecured in the final microstructure (the microstructure of the formedproduct). Also, when the heating temperature of the steel sheet exceedsAc₃ transformation point−20° C., the transformation amount to austeniteincreases excessively in heating, and a predetermined amount of annealedmartensite and annealed bainite cannot be secured in the finalmicrostructure (the microstructure of the formed product).

In order to make austenite formed in the heating step described above adesired microstructure while preventing formation of the microstructuresuch as ferrite or pearlite, it is necessary to properly control theaverage cooling rate and the cooling finishing temperature duringforming and after forming. From such a viewpoint, it is necessary tomake the average cooling rate during forming 20° C./s or more and tomake the cooling finishing temperature a temperature or below, thetemperature being lower than the bainite transformation startingtemperature Bs by 100° C. The average cooling rate during forming ispreferably 30° C./s or more (more preferably 40° C./s or more). Bytransforming austenite having been present in heating to bainite andmartensite while preventing formation of the microstructure such asferrite or martensite by making the cooling finishing temperature atemperature equal to or below the bainite transformation startingtemperature Bs, fine austenite is made remain between the laths ofbainite and martensite, and a predetermined amount of retained austeniteis secured while securing bainite and martensite.

When the cooling finishing temperature becomes higher than thetemperature that is lower than the bainite transformation startingtemperature Bs by 100° C. and the average cooling rate is less than 20°C./s, the microstructure such as ferrite, pearlite and the like isformed, a predetermined amount of retained austenite cannot be secured,and elongation (ductility) in the formed product deteriorates.

Although control of the average cooling rate basically becomesunnecessary at the stage the temperature becomes equal to or below thetemperature lower than the bainite transformation starting temperatureBs by 100° C., cooling may be executed to the room temperature with theaverage cooling rate of 1° C./s or more and 100° C./s or less forexample. Also, control of the average cooling rate during forming andafter completion of forming can be achieved by means such as (a) tocontrol the temperature of the forming tool (the cooling medium shown inFIG. 1 above), and (b) to control the thermal conductivity of the tool.

In the press-formed product manufactured by hot press forming asdescribed above, the metal microstructure is formed of retainedaustenite: 3-20 area %, annealed martensite and/or annealed bainite:30-87 area %, and martensite as quenched: 10-67 area %, the carbonamount in the retained austenite is 0.60% or more, and the balance ofhigh strength and elongation can be achieved with a high level and as auniform property within the formed product. The reasons for setting therange of each requirement (the basic microstructure and the carbonamount in the retained austenite) in such a hot press-formed product areas described below.

Retained austenite has an effect of increasing the work hardening ratio(transformation induced plasticity) and improving ductility of thepress-formed product by being transformed to martensite during plasticdeformation. In order to exert such an effect, the fraction of retainedaustenite should be made 3 area % or more. Ductility becomes moreexcellent as the fraction of retained austenite is higher. In thecomposition used for a steel sheet for an automobile, retained austenitethat can be secured is limited, and approximately 20 area % becomes theupper limit. Preferable lower limit of retained austenite is 5 area % ormore (more preferably 7 area % or more).

By making the main microstructure annealed martensite and/or annealedbainite which is fine and has low dislocation density, ductility(elongation) of the press-formed product can be enhanced while securinga predetermined strength. From such a viewpoint, the fraction ofannealed martensite and/or annealed bainite is made 30 area % or more.However, when this fraction exceeds 87 area %, the fraction of retainedaustenite becomes insufficient, and ductility (residual ductility)deteriorates. Preferable lower limit of annealed martensite and/orannealed bainite is 40 area % or more (more preferably 50 area % ormore), and preferable upper limit is less than 80 area % (morepreferably less than 70 area %).

Because martensite as quenched is a microstructure inferior inductility, when much amount thereof is present, elongation isdeteriorated, however, in order to achieve high strength of over 100kg/mm² class in a microstructure with low matrix strength such asannealed martensite, it is necessary to secure a predetermined amount ofmartensite as quenched. From such a viewpoint, the fraction ofmartensite as quenched is made 10 area % or more. However, when thefraction of martensite as quenched increases excessively, strengthincreases excessively and elongation becomes insufficient, and thereforethe fraction thereof should be 67 area % or less. Preferable lower limitof the fraction of martensite as quenched is 20 area % or more (morepreferably 30 area % or more), and preferable upper limit is 60 area %or less (more preferably 50 area % or less).

With respect to the microstructure other the above, ferrite, pearlite,bainite and the like may be included as the remainder microstructure,however, these microstructures are inferior in contribution to strengthand contribution to ductility compared to other microstructures, and itis basically preferable not to be contained (it may also be 0 area %).However, up to 20 area % is allowable. The remainder microstructure ispreferably 10 area % or less, more preferably 5 area % or less.

The carbon amount in retained austenite affects the timing of workinduced transformation of retained austenite to martensite at the timeof deformation such as the tensile test and the like, and enhances thetransformation induced plasticity (TRIP) effect by causing the workinduced transformation at a higher strain zone as the carbon amount ishigher. In the case of the process of the present invention, carbon isdischarged during cooling from the martensite lath formed to surroundingaustenite. At that time, if Ti-carbide or carbonitride dispersed insteel is dispersed coarsely, growth of the martensite lath in thelongitudinal direction proceeds without being impeded, and therefore themartensite lath narrow in the width, long, and having a large aspectratio is obtained. As a result, carbon is easily discharged from themartensite lath to the width direction, the carbon amount in retainedaustenite increases, and the ductility improves. From such a viewpoint,in the press-formed product of the present invention, the carbon amountin retained austenite in steel was stipulated to be 0.60% or more.Further, although the carbon amount in retained austenite can beconcentrated to approximately 0.70%, approximately 1.0% is the limit.

When the steel sheet for hot pressing use of the present invention isused, by properly adjusting the press forming condition (heatingtemperature and cooling rate), the properties such as strength,elongation and the like of the press-formed product can be controlled,the press-formed product with high ductility (residual ductility) isobtained, and therefore application to a portion (energy absorptionmember for example) to which it has been difficult to apply conventionalpress-formed products becomes also possible which is very useful inexpanding the application range of the press-formed product. Also, notonly the single region formed product described above, a press-formedproduct exerting strength-ductility balance according to each region(may be hereinafter referred to as “plural region formed product”) isobtained when the heating temperature and the condition of each regionin forming are properly controlled and the microstructure of each regionis adjusted in manufacturing the press-formed product by press formingof a steel sheet using a press-forming tool.

The plural region formed product can be manufactured as described aboveusing the steel sheet for hot pressing use of the present invention bydividing a heating region of the steel sheet into at least two regions,heating one region thereof (hereinafter referred to as the first region)to a temperature of Ac₃ transformation point or above and 950° C. orbelow, heating another region (hereinafter referred to as the secondregion) to a temperature of Ac₁ transformation point+20° C. or above andAc₃ transformation point−20° C. or below, thereafter starting pressforming of both of the first and second regions, and executing coolingto a temperature of martensite transformation starting temperature Ms orbelow while securing the average cooling rate of 20° C./s or more withina tool in both of the first and second regions during forming and afterforming.

According to the method described above, by dividing the heating regionof the steel sheet into at least two regions (high strength side regionand low strength side region) and controlling the manufacturingcondition according to each region, such a press-formed product thatstrength-ductility balance according to each region is exerted isobtained. The second region out of two regions corresponds to the lowstrength side region, and the manufacturing condition, microstructureand properties in this region is basically same to those of the singleregion formed product described above. Below, the manufacturingcondition for forming the other first region (corresponding to the highstrength side region) will be described. Also, in executing thismanufacturing method, it is required to form regions with differentheating temperature by a single steel sheet, however, by using anexisting heating furnace (for example, far infrared furnace, electricfurnace+shield), controlling while making the boundary section of thetemperature 50 mm or less is possible.

(Manufacturing Condition of the First Region/High Strength Side Region)

In order to properly adjust the microstructure of the press-formedproduct, it is necessary to control the heating temperature to apredetermined range. By properly controlling this heating temperature,transformation to a microstructure mainly of martensite is caused whilesecuring a predetermined amount of retained austenite in the coolingstep after heating, and a desired microstructure can be achieved withinthe range of the final hot press-formed product. When the steel sheetheating temperature in this region is below Ac₃ transformation point, asufficient amount of austenite cannot be obtained in heating, and apredetermined amount of retained austenite cannot be secured in thefinal microstructure (the microstructure of the formed product). Also,when the heating temperature of the steel sheet exceeds 950° C., thegrain size of austenite becomes large in heating, martensitetransformation starting temperature (Ms point) and martensitetransformation finishing temperature (Mf point) rise, retained austenitecannot be secured in quenching, and excellent formability is notachieved. The heating temperature of the steel sheet is preferably Ac₃transformation point+50° C. or above and 900° C. or below.

In order to make austenite formed in the heating step described above adesired microstructure while preventing formation of the microstructuresuch as ferrite or pearlite, it is necessary to properly control theaverage cooling rate and the cooling finishing temperature duringforming and after forming. From such a viewpoint, the average coolingrate during forming should be 20° C./s or more and the cooling finishingtemperature should be martensite transformation starting temperature (Mspoint) or below. The average cooling rate during forming is preferably30° C./s or more (more preferably 40° C./s or more). By transformingaustenite having been present in heating to martensite while preventingformation of the microstructure such as ferrite or pearlite by makingthe cooling finishing temperature the martensite transformation startingtemperature (Ms point) or below, martensite is secured. Specifically,the cooling finishing temperature is 400° C. or below, preferably 300°C. or below.

In the press-formed product obtained by such a method, the metalmicrostructure, precipitates and the like are different between thefirst region and the second region. In the first region, the metalmicrostructure is of retained austenite: 3-20 area % (the action andeffect of retained austenite are same to the above), and martensite: 80area % or more. In the second region, the metal microstructure same tothat of the single region formed product described above and 0.60% ormore of the carbon amount in retained austenite are fulfilled.

By making the main microstructure of the first region martensite withhigh strength containing a predetermined amount of retained austenite,ductility and high strength in a specific region in the hot press-formedproduct can be secured. From such a viewpoint, the area fraction ofmartensite should be 80 area % or more. The fraction of martensite ispreferably 85 area % or more (more preferably 90 area % or more). Also,as the microstructure in the first region, ferrite, pearlite, bainiteand the like may be included in a part thereof.

Although the effect of the present invention will be shown below morespecifically by examples, the examples described below do not limit thepresent invention, and any of the design alterations judging from thepurposes described above and below is to be included in the technicalrange of the present invention.

EXAMPLES Example 1

Steel (steel Nos. 1-32) having the chemical component composition shownin Table 1 below was molten in vacuum, was made a slab for experiment,was thereafter made a steel sheet by hot rolling, was thereafter cooled,and was subjected to a treatment that simulates winding (sheetthickness: 3.0 mm). The winding simulated treatment method includedcooling to the winding temperature, putting the sample thereafter into afurnace heated to the winding temperature, holding for 30 min, andcooling in the furnace. The manufacturing condition for the steel sheetat that time is shown in Table 2 below. Also, Ac₁ transformation point,Ac₃ transformation point, Ms point, and Bs point in Table 1 wereobtained using the formula (2)-formula (5) below (refer to “The physicalMetallurgy of Steels”, Leslie, Maruzen Company, Limited (1985) forexample). Also, the treatments (1)-(3) shown in the remarks column inTable 2 express that each treatment (rolling, cooling, alloying) shownbelow was executed.

Ac₁ transformation point (°C.)=723+29.1×[Si]−10.7×[Mn]+16.9×[Cr]−16.9×[Ni]  (2)

Ac₃ transformation point (°C.)=910−203×[C]^(1/2)+44.7×[Si]−30×[Mn]+700×[P]+400×[Al]+400×[Ti]+104×[V]−11×[Cr]+31.5×[Mo]−20×[Cu]−15.2×[Ni]  (3)

Ms point (°C.)=550−361×[C]−39×[Mn]−10×[Cu]−17×[Ni]−20×[Cr]−5×[Mo]+30×[Al]  (4)

Bs point (° C.)=830−270×[C]−90×[Mn]−37×[Ni]−70×[Cr]−83×[Mo]  (5)

wherein [C], [Si], [Mn], [P], [Al], [Ti], [V], [Cr], [Mo], [Cu] and [Ni]represent the content (mass %) of C, Si, Mn, P, Al, Ti, V, Cr, Mo, Cuand Ni respectively. Also, when the element shown in each term of theformulae (2)-(5) above is not contained, calculation is done assumingthat the term is null.

Treatment (1): After finish rolling, cooling was executed to 650° C.with the average cooling rate of 50° C./s, cooling was thereafterexecuted for 10 s from 650° C. with the average cooling rate of 5° C./s,and cooling was thereafter executed to the winding temperature with theaverage cooling rate of 50° C./s. The front and back surfaces werethereafter polished and the thickness was reduced to 1.6 mm so as tomatch the thickness to that of the treatments (2) and (3).

Treatment (2): The hot-rolled steel sheet was cold-rolled, was heatedthereafter to 860° C. simulating continuous annealing, was cooledthereafter to 400° C. with the average cooling rate of 30° C./s, and washeld.

Treatment (3): The hot-rolled steel sheet was cold-rolled, was heatedthereafter to 860° C. for simulating continuous hot dip galvanizingline, was cooled thereafter to 400° C. with the average cooling rate of30° C./s, was held, was thereafter heated further by (500° C.×10 s), andwas cooled thereafter.

TABLE 1 Steel Chemical component composition* (mass %) No. C Si Mn P SAl B Ti N V Nb 1 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040— — 2 0.150 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 — — 30.220 0.05 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 — — 4 0.220 0.501.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 — — 5 0.220 1.20 1.200.0050 0.0020 0.030 0.0020 0.044 0.0040 — — 6 0.220 1.20 1.20 0.00500.0020 0.030 0.0020 0.044 0.0040 — — 7 0.220 1.20 1.20 0.0050 0.00200.030 0.0020 0.044 0.0040 — — 8 0.220 1.20 1.20 0.0050 0.0020 0.0300.0020 0.044 0.0040 — — 9 0.220 1.20 1.20 0.0050 0.0020 0.030 0.00200.044 0.0040 — — 10 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.0440.0040 — — 11 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 —— 12 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 — — 130.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 — — 14 0.2201.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 — — 15 0.220 2.00 1.200.0050 0.0020 0.030 0.0020 0.044 0.0040 — — 16 0.350 1.20 1.20 0.00500.0020 0.030 0.0020 0.044 0.0040 — — 17 0.450 1.20 1.20 0.0050 0.00200.030 0.0020 0.044 0.0040 — — 18 0.720 1.20 1.20 0.0050 0.0020 0.0300.0020 0.044 0.0040 — — 19 0.220 1.20 0.80 0.0050 0.0020 0.030 0.00200.044 0.0040 — — 20 0.220 1.20 2.40 0.0050 0.0020 0.030 0.0020 0.0440.0040 — — 21 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.100 0.0040 —— 22 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.200 0.0040 — — 230.220 0.50 1.20 0.0050 0.0020 0.40 0.0020 0.044 0.0040 — — 24 0.220 1.201.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 0.030 — 25 0.220 1.20 1.200.0050 0.0020 0.030 0.0020 0.044 0.0040 — 0.020 26 0.220 1.20 1.200.0050 0.0020 0.030 0.0020 0.044 0.0040 — — 27 0.220 1.20 1.20 0.00500.0020 0.030 0.0020 0.044 0.0040 — — 28 0.220 1.20 1.20 0.0050 0.00200.030 0.0020 0.044 0.0040 — — Chemical component Steel composition*(mass %) Ac₃ − Ac₁ + 20° C. Bs − Ms point No. Cu Ni Cr Mo 20° C. (° C.)(° C.) 100° C. (° C.) (° C.) 1 — — — — 845 765 563 425 2 — — 0.20 — 860768 568 446 3 — — 0.20 — 792 735 549 421 4 — — 0.20 — 812 748 549 421 5— — 0.20 — 843 768 549 421 6 — — 0.20 — 843 768 549 421 7 — — 0.20 — 843768 549 421 8 — — 0.20 — 843 768 549 421 9 — — 0.20 — 843 768 549 421 10— — 0.20 — 843 768 549 421 11 — — 0.20 — 843 768 549 421 12 — — 0.20 —843 768 549 421 13 — — 0.20 — 843 768 549 421 14 — — 0.20 — 843 768 549421 15 — — 0.20 — 879 792 549 421 16 — — 0.20 — 818 768 514 374 17 — —0.20 — 802 768 487 338 18 — — 0.20 — 766 768 414 240 19 — — 0.20 — 855773 585 436 20 — — 0.20 — 807 756 441 374 21 — — 0.20 — 866 768 549 42122 — — 0.20 — 906 768 549 421 23 — — 0.20 — 960 748 549 432 24 — — 0.20— 846 768 549 421 25 — — 0.20 — 843 768 549 421 26 0.20 — 0.20 — 839 768549 419 27 — 0.20 0.20 — 840 765 541 417 28 — — 0.20 0.20 849 768 532420 *The remainder: iron and inevitable impurities other than P, S, N.

TABLE 2 Steel sheet manufacturing condition Steel Heating Finish rollingCooling time Average cooling rate Winding No. temperature (° C.)temperature (° C.) of 750-700° C. (s) of 700° C.-450° C. (° C./s)temperature (° C.) Remarks 1 1200 800 15 50 200 — 2 1200 800 15 50 200 —3 1200 800 15 50 200 — 4 1200 800 15 50 200 — 5 1200 800 15 50 200 — 61200 800 15 50 200 — 7 1200 800 15 50 200 — 8 1200 800 15 50 200 — 91200 800 1 17 200 Treatment (1) 10 1200 900 1 17 450 Treatment (1) 111200 800 15 50 400 Cold rolling: 20% 12 1200 800 15 50 400 Treatment (2)13 1200 800 15 50 400 Treatment (3) 14 1200 800 15 50 580 — 15 1200 80015 50 200 — 16 1200 800 15 50 200 — 17 1200 800 15 50 200 — 18 1200 80015 50 200 — 19 1200 800 15 50 200 — 20 1200 800 15 50 200 — 21 1200 80015 50 200 — 22 1200 800 15 50 200 — 23 1200 800 15 50 200 — 24 1200 80015 50 200 — 25 1200 800 15 50 200 — 26 1200 800 15 50 200 — 27 1200 80015 50 200 — 28 1200 800 15 50 200 —

With respect to the steel sheet obtained, analysis of the precipitationstate of Ti and observation of the metal microstructure (the fraction ofeach microstructure) were executed by the procedure described below. Theresult is shown in Table 3 below along with the calculated value of0.5×[total Ti amount(mass %)−3.4[N]] (shown as 0.5×(total Tiamount-3.4[N])).

[Analysis of Precipitation State of Ti of Steel Sheet]

An extraction replica sample was prepared, and a transmission electronmicroscope image (magnifications: 100,000 times) of Ti-containingprecipitates was photographed using a transmission electron microscope(TEM). At this time, by composition analysis of the precipitates usingan energy dispersion type X-ray spectrometer (EDX), Ti-containingprecipitates were identified. The area of the Ti-containing precipitatesof at least 100 pieces was measured by image analysis, those having theequivalent circle diameter of 30 nm or less were extracted, and theaverage value thereof was made the size of the precipitates. Also, inthe table, the size is shown as “average equivalent circle diameter ofTi-containing precipitates”. Further, with respect to precipitated Tiamount (mass %)−3.4[N] (the Ti amount present as the precipitates),extraction residue analysis (in extraction treatment, the precipitatescoagulate, and fine precipitates also can be measured) was executedusing a mesh with mesh diameter: 0.1 μm, and precipitated Ti amount(mass %)−3.4[N] (expressed as “precipitated Ti amount-3.4[N]” in Table3) was obtained. Also, when the Ti-containing precipitates partlycontained V and Nb, the contents of these precipitates were alsomeasured.

[Observation of Metal Microstructure (Fraction of Each Microstructure)]

(1) With respect to the microstructure of martensite and bainite in thesteel sheet, the steel sheet was corroded by nital, martensite andbainite were distinguished from each other by SEM observation(magnifications: 1,000 times or 2,000 times), and each fraction (arearatio) was obtained.

(2) The retained austenite fraction in the steel sheet was measured byX-ray diffraction method after the steel sheet was ground up to ¼thickness thereof and was thereafter subjected to chemical polishing(for example, ISJJ Int. Vol. 33. (1933), No. 7, P. 776).

TABLE 3 Steel sheet for press forming use Average equivalentPrecipitated 0.5 × (total circle diameter of Steel Ti amount − 3.4[N] Tiamount − 3.4[N]) Ti-containing Fraction of Fraction of No. (mass %)(mass %) precipitates (nm) martensite (area %) bainite (area %) Others 10.024 0.015 10.0 100 0 — 2 0.024 0.015 10.0 100 0 — 3 0.025 0.015 10.0100 0 — 4 0.027 0.015 10.0 100 0 — 5 0.029 0.015 10.0 100 0 — 6 0.0260.015 10.0 100 0 — 7 0.023 0.015 10.0 100 0 — 8 0.026 0.015 10.0 100 0 —9 0.006 0.015 5.0 100 0 — 10 0.012 0.015 2.0 0 100 — 11 0.026 0.015 10.00 100 — 12 0.030 0.015 10.0 0 100 — 13 0.027 0.015 10.0 0 100 — 14 0.0250.015 10.0 0 0 Ferrite + Pearlite: 100% 15 0.028 0.015 10.0 100 0 — 160.023 0.015 10.0 90 0 Retained austenite 10% 17 0.026 0.015 10.0 80 0Retained austenite 20% 18 0.030 0.015 10.0 60 0 Retained austenite 40%19 0.023 0.015 10.0 100 0 — 20 0.023 0.015 10.0 100 0 — 21 0.084 0.04310.0 100 0 — 22 0.144 0.093 18.0 100 0 — 23 0.029 0.015 10.0 100 0 — 240.024 0.015 10.0 100 0 — 25 0.024 0.015 10.0 100 0 — 26 0.026 0.015 10.0100 0 — 27 0.028 0.015 10.0 100 0 — 28 0.024 0.015 10.0 100 0 —

Each steel sheet described above (1.6 mm^(t)×150 mm×200 mm) (withrespect to those other than the treatments of (1)-(3) described above,the thickness was adjusted to 1.6 mm by hot rolling) was heated to apredetermined temperature in a heating furnace, and was thereaftersubjected to press forming and cooling treatment using the tool (FIG. 1above) of a hat shape to obtain the press-formed product. The pressforming conditions (heating temperature, average cooling rate, and rapidcooling finishing temperature in press forming) are shown in Table 4below.

TABLE 4 Press forming condition Steel Heating Average cooling Rapidcooling finishing No. temperature (° C.) rate (° C./s) temperature (°C.) 1 790 40 300 2 800 40 300 3 760 40 300 4 770 40 300 5 810 40 300 6950 40 300 7 820 5 300 8 800 40 600 9 800 40 300 10 810 40 300 11 800 40300 12 820 40 300 13 810 40 300 14 800 40 300 15 830 40 300 16 790 40300 17 790 40 300 18 770 40 300 19 800 40 300 20 780 40 300 21 810 40300 22 820 40 300 23 830 40 300 24 800 40 300 25 810 40 300 26 800 40300 27 810 40 300 28 820 40 300

With respect to the formed product obtained, tensile strength (TS),elongation (total elongation EL), and observation of the metalmicrostructure (the fraction of each microstructure) were measured bymethods described below.

[Measurement of Tensile Strength (TS) and Elongation (Total ElongationEL)]

The tensile test was executed using JIS No. 5 test specimen, and thetensile strength (TS) and the elongation (EL) were measured. At thistime, the strain rate of the tensile test was made 10 mm/s. In thepresent invention, the case 980-1,179 MPa of the tensile strength (TS)and 20% or more of the elongation (EL) were satisfied and thestrength-elongation balance (TS×EL) was 24,000 (MPa·%) or more wasevaluated to have passed.

(Observation of Metal Microstructure (Fraction of Each Microstructure))

(1) With respect to the microstructure of annealed martensite, bainiteand annealed bainite in the steel sheet, the steel sheet was corroded bynital, annealed martensite, bainite and annealed bainite weredistinguished from each other by SEM observation (magnifications: 1,000times or 2,000 times), and each fraction (area ratio) was obtained.

(2) The retained austenite fraction in the steel sheet was measured byX-ray diffraction method after the steel sheet was ground up to ¼thickness thereof and was thereafter subjected to chemical polishing(for example, ISJJ Int. Vol. 33. (1933), No. 7, P. 776). At this time,the carbon amount in retained austenite was also measured.

(3) With respect to the fraction of martensite as quenched, the steelsheet was LePera-corroded, the area ratio of the white contrast wasmeasured as the mixture microstructure of martensite as quenched andretained austenite, the retained austenite fraction obtained by X-raydiffraction was deducted therefrom, and the fraction of martensite asquenched was calculated.

The observation results (fraction of each microstructure) of the metalmicrostructure are shown in Table 5 below. Also, the mechanicalproperties (tensile strength TS, elongation EL, and TS×EL) of the formedproduct are shown in Table 6 below.

TABLE 5 Metal microstructure of formed product Fraction of annealedmartensite Fraction of martensite Fraction of retained Carbon amount inretained Steel No. and/or annealed bainite (area %) as quenched (area %)austenite (area %) austenite (mass %) Others 1 70 23 7 0.65 — 2 67 23 70.65 Ferrite 3% 3 70 28 2 0.45 — 4 70 25 5 0.65 — 5 70 23 7 0.65 — 6 7023 7 0.65 — 7 70 2 0 0.65 Pearlite 18%, Ferrite 10% 8 70 6 3 0.65Pearlite 12%, Ferrite 9% 9 70 23 7 0.65 — 10 70 23 7 0.65 — 11 70 23 70.65 — 12 70 23 7 0.65 — 13 70 23 7 0.65 — 14 — 33 7 0.52 Ferrite 60% 1570 23 9 0.65 — 16 60 29 11 0.65 — 17 55 32 13 0.65 — 18 16 62 22 0.65 —19 70 25 5 0.68 — 20 70 21 9 0.61 — 21 70 23 7 0.65 — 22 70 23 7 0.65 —23 70 23 7 0.65 — 24 70 23 7 0.65 — 25 70 23 7 0.65 — 26 70 23 7 0.65 —27 70 23 7 0.65 — 28 70 23 7 0.65 —

TABLE 6 Mechanical properties of formed product Steel Tensile strengthElongation EL TS × EL No. TS (MPa) (%) (MPa · %) 1 1074 23.5 25239 21022 26.1 26674 3 1014 12.0 26364 4 983 25.5 25067 5 1016 26.2 26619 61524 11.3 17221 7 781 19.7 15386 8 856 19.0 16264 9 1026 22.6 23188 101044 22.0 22968 11 1051 25.9 27221 12 1018 25.3 25755 13 1039 24.1 2504014 1063 19.9 21154 15 1038 25.6 26573 16 1192 23.5 28012 17 1260 28.035280 18 1321 10.2 13474 19 1031 24.3 25053 20 1021 29.0 29609 21 104525.9 27066 22 1045 18.1 18915 23 1013 26.1 26439 24 1049 24.0 25176 251033 26.0 26858 26 1007 26.1 26283 27 1059 25.2 26687 28 1045 24.2 25289

From these results, following consideration can be made. Those of thesteel Nos. 1, 2, 4, 5, 11-13, 15-17, 19-21, 23-32 are examplesfulfilling the requirements stipulated in the present invention, and itis known that components excellent in strength-elongation balance havebeen obtained.

On the other hand, those of the steel Nos. 3, 6-10, 14, 18, 22 are thecomparative examples not fulfilling any of the requirements stipulatedin the present invention, and any of the properties is deteriorated.That is, in that of the steel No. 3, a steel sheet with low Si contentis used, the fraction of retained austenite in the formed product is notsecured, the carbon amount in the retained austenite drops, and theelongation is not enough. In that of the steel No. 6, the heatingtemperature in forming is high, only low elongation EL is obtained, andthe strength-elongation balance (TS×EL) also deteriorates.

In that of the steel No. 7, the average cooling rate in press forming isslow, pearlite and ferrite are formed, the fraction of martensite asquenched cannot be secured, and the strength-elongation balance (TS×EL)is deteriorated. In that of the steel No. 8, the rapid cooling finishingtemperature is high, pearlite and ferrite are formed, the fraction ofmartensite as quenched cannot be secured, only low elongation isobtained, and the strength-elongation balance (TS×EL) is alsodeteriorated.

In those of the steel Nos. 9, 10, the condition in manufacturing thesteel is not appropriate, the amount of precipitated Ti is insufficient(steel Nos. 9, 10), Ti-containing precipitates are small (steel No. 10),and, when press forming is executed using such a steel sheet, thestrength-elongation balance (TS×EL) is deteriorated even if the formingcondition is appropriate.

In that of the steel No. 14, a steel sheet whose metal microstructure isof ferrite+pearlite of 100 area % which is caused by the windingtemperature is used, the fraction of annealed martensite and/or annealedbainite during forming cannot be secured, and the strength-elongationbalance (TS×EL) is deteriorated. In that of the steel No. 18, the steelsheet with excessive C content is used, the strength becomes high, andonly low elongation EL is obtained. In that of the steel No. 22, thesteel sheet with excessive Ti content is used, and thestrength-elongation balance (TS×EL) is deteriorated.

Example 2

Steel (steel Nos. 33-37) having the chemical component composition shownin Table 7 below was molten in vacuum, was made a slab for experiment,was thereafter hot-rolled, and was thereafter cooled and wound (sheetthickness: 3.0 mm). The manufacturing condition for the steel sheet atthat time is shown in Table 8 below.

TABLE 7 Ac₃ − Ac₁ + Bs − Ms Steel Chemical component composition* (mass%) 20° C. 20° C. 100° C. point No. C Si Mn P S Al B Ti N V Nb Cu Ni CrMo (° C.) (° C.) (° C.) (° C.) 29 0.220 1.20 1.20 0.0050 0.0020 0.0300.0020 0.044 0.0040 — — — — 0.20 — 843 768 549 421 30 0.220 1.20 1.200.0050 0.0020 0.030 0.0020 0.044 0.0040 — — — — 0.20 — 843 768 549 42131 0.350 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.044 0.0040 — — — — 0.20— 818 768 514 374 32 0.220 1.20 1.20 0.0050 0.0020 0.030 0.0020 0.0440.0040 — — — — 0.20 — 843 768 549 421 *The remainder: iron andinevitable impurities other than P, S, N.

TABLE 8 Steel sheet manufacturing condition Heating Steel temperatureFinish rolling Cooling time Average cooling rate Winding No. (° C.)temperature (° C.) of 750-700° C. (s) of 700° C.-450° C. (° C./s)temperature (° C.) Remarks 29 1200 800 15 50 200 — 30 1200 800 15 50 200— 31 1200 800 15 50 200 — 32 1200 800 1 17 200 Treatment (1)

With respect to the steel sheet obtained, analysis of the precipitationstate of Ti-containing precipitates and observation of the metalmicrostructure (the fraction of each microstructure) were executedsimilarly to Example 1. The result is shown in Table 9 below.

TABLE 9 Steel sheet for press forming use Precipitated 0.5 × (totalAverage equivalent circle Fraction of Steel Ti amount − 3.4[N] Ti amount− 3.4[N]) diameter of Ti-containing martensite Fraction of No. (mass %)(mass %) precipitates (nm) (area %) bainite (area %) Others 29 0.0300.015 10.0 100 0 — 30 0.023 0.015 10.0 100 0 — 31 0.028 0.015 10.0 90 0Retained austenite 10% 32 0.006 0.015 2.3 100 0 —

Each steel sheet described above (3.0 mm^(t)×150 mm×200 mm) was heatedto a predetermined temperature in a heating furnace, and was subjectedthereafter to press forming and cooling treatment using the tool (FIG. 1above) of a hat shape to obtain the formed product. At this time, thesteel sheet was put in an infrared furnace and the portion intended tobe high-strengthened (the steel sheet portion corresponding to the firstregion) was configured so that infrared rays directly hit so as to allowhigh temperature heating, whereas the portion intended to below-strengthened (the steel sheet portion corresponding to the secondregion) was shielded with a cover so that a part of the infrared rayswas blocked so as to allow low temperature heating, and thereby theheating temperature was differentiated. Therefore, the formed producthas the regions with different strength within a single component. Thepress forming conditions (heating temperature, average cooling rate, andrapid cooling finishing temperature of each region in press forming) areshown in Table 10 below.

TABLE 10 Press forming condition Heating Average Rapid cooling Steeltemperature cooling finishing No. Region (° C.) rate (° C./s)temperature (° C.) 29 Low strength side 810 40 300 High strength side920 40 300 30 Low strength side 800 40 300 High strength side 850 40 30031 Low strength side 790 40 300 High strength side 920 40 300 32 Lowstrength side 810 40 300 High strength side 920 40 300

With respect to the formed product obtained, tensile strength (TS),elongation (total elongation EL), observation of the metalmicrostructure (the fraction of each microstructure), and the carbonamount in retained austenite in each region were obtained similarly toExample 1.

The observation results (fraction of each microstructure) of the metalmicrostructure are shown in Table 11 below. Also, the mechanicalproperties (tensile strength TS, elongation EL, and TS×EL) of the formedproduct are shown in Table 12 below. Further, the case 1,470 MPa or moreof the tensile strength (TS) and 8% or more of the elongation (EL) werefulfilled and the strength-elongation balance (TS×EL) was 14,000 (MPa·%)or more on the high strength side was evaluated to have passed (theevaluation criteria of the low strength side are same to those ofExample 1).

TABLE 11 Metal microstructure of formed product Fraction Carbon amountSteel Fraction of annealed martensite Fraction of martensite of retainedin retained No. Region and/or annealed bainite (area %) as quenched(area %) austenite (area %) austenite (mass %) Others 29 Low strengthside 70 23 7 0.65 — High strength side 0 94 6 0.55 — 30 Low strengthside 70 23 7 0.65 — High strength side 0 61 6 0.52 Ferrite 33% 31 Lowstrength side 60 29 11 0.65 — High strength side — 95 5 0.53 — 32 Lowstrength side 70 23 7 0.65 — High strength side 0 94 6 0.54 —

TABLE 12 Mechanical properties of formed product Steel Tensile strengthElongation EL TS × EL No. Region TS (MPa) (%) (MPa · %) 29 Low strengthside 1058 24.1 25498 High strength side 1511 11.0 16621 30 Low strengthside 1063 23.9 25406 High strength side 1278 13.0 16614 31 Low strengthside 1192 25.0 29800 High strength side 1820 10.1 18382 32 Low strengthside 1049 21.5 22554 High strength side 1499 11.0 16489

From this result, following consideration can be made. Those of thesteel Nos. 33, 35, 37 are examples fulfilling the requirementsstipulated in the present invention, and it is known that componentsexcellent in the strength-elongation balance in each region have beenobtained.

On the other hand, those of the steel Nos. 34, 36 are the comparativeexamples not fulfilling any of the requirements stipulated in thepresent invention, and any of the properties is deteriorated. That is,in that of the steel No. 34, the heating temperature in press forming islow, and the strength on the high strength side drops. In that of thesteel No. 36, a steel sheet with small size of Ti-containingprecipitates is used, only low strength is obtained on the high strengthside, and the strength-elongation balance (TS×EL) is deteriorated on thelow strength side.

Although the present invention has been described in detail andreferring to specific embodiments, it is obvious for a person with anordinary skill in the art that various alterations and amendments can beeffected without departing from the spirit and the range of the presentinvention.

The present application is based on Japanese Patent Application(JP-A-No. 2012-053844) applied on Mar. 9, 2012, and the contents thereofare hereby incorporated by reference.

INDUSTRIAL APPLICABILITY

The present invention is suitable to a steel sheet for hot pressing usethat is used in manufacturing structural components of an automobile.

REFERENCE SIGNS LIST

-   -   1 . . . punch    -   2 . . . die    -   3 . . . blank holder    -   4 . . . steel sheet (blank)

1. A steel sheet, comprising, in mass %, with respect to a chemicalcomponent composition: C: 0.15-0.5%; Si: 0.2-3%; Mn: 0.5-3%; P: 0.05% orless, exclusive of 0%; S: 0.05% or less, exclusive of 0%; Al: 0.01-1%;B: 0.0002-0.01%; Ti: 3.4[N]+0.01% or more and 3.4[N]+0.1% or less,wherein [N] represents N content in mass %; and N: 0.001-0.01%respectively, with a remainder comprising iron and inevitableimpurities, wherein some of Ti-containing precipitates contained in thesteel sheet, each of which having an equivalent circle diameter of 30 nmor less, have an average equivalent circle diameter of 3 nm or more, aprecipitated Ti amount and a total Ti amount in the steel fulfill arelationship of formula (1), and a sum total of a fraction of bainiteand a fraction of martensite in a metal microstructure is 80 area % ormore:precipitated Ti amount (mass %)−3.4[N]>0.5×[(total Ti amount (mass%))−3.4[N]]  (1) wherein [N] represents the content (mass %) of N in thesteel.
 2. The steel sheet according to claim 1, further comprising atleast one of (a)-(c) below as other elements: (a) at least one elementselected from the group consisting of V, Nb and Zr by 0.1% or less,exclusive of 0%, in total; (b) at least one element selected from thegroup consisting of Cu, Ni, Cr and Mo by 1% or less, exclusive of 0%, intotal; and (c) at least one element selected from the group consistingof Mg, Ca and REM by 0.01% or less, exclusive of 0%, in total.
 3. Amethod for manufacturing a press-formed product, comprising: hotpressing with a steel sheet according to claim 1; heating the steelsheet to a temperature of Ac₁ transformation point+20° C. or above andAc₃ transformation point−20° C. or below; thereafter starting pressforming; and executing cooling to a temperature or below, thetemperature being lower than a bainite transformation startingtemperature Bs by 100° C., while securing an average cooling rate of 20°C./s or more within a tool during forming and after completion offorming.
 4. A press-formed product obtained by the method according toclaim 3, wherein the metal microstructure comprises retained austenite:3-20 area %, annealed martensite and/or annealed bainite: 30-87 area %,and martensite as quenched: 10-67 area %, and an amount of carbon in theretained austenite is 0.60% or more.
 5. A method for manufacturing apress-formed product, comprising: hot pressing with the steel sheetaccording to claim 1; dividing a heating region of the steel sheet intotwo regions; heating one region thereof to a temperature of Ac₃transformation point or above and 950° C. or below; heating the otherregion to a temperature of Ac₁ transformation point+20° C. or above andAc₃ transformation point−20° C. or below; thereafter starting pressforming; and executing cooling to a temperature of martensitetransformation starting temperature Ms or below while securing anaverage cooling rate of 20° C./s or more within a tool during formingand after completion of forming.
 6. A press-formed product obtained bythe method according to claim 5, comprising a first region whose metalmicrostructure comprises retained austenite: 3-20 area % and martensite:80 area % or more, and a second region whose metal microstructurecomprises retained austenite: 3-20 area %, annealed martensite and/orannealed bainite: 30-87 area %, and martensite as quenched: 10-67 area %with an amount of carbon in the retained austenite being 0.60% or more.